How much dV am I wasting using a 605km parking orbit?

[The Ideal Gas Lawyer]

When I'm setting up a burn to another planet I will generally head into an orbit that lets me use the 1Mx time warp while I wait for the correct phase angles for the transfer. I know the Oberth effect makes this less efficient, but by how much?

Edited December 12, 2013 by The Ideal Gas Lawyer

[Taki117]

That's quite the large parking orbit. I don't know how much the Oberth effect would change, but I do know that if you set your orbit at about 200km and then set a pod on the pad (Or have a flag placed at the pad) you can switch to map view and warp all you want (While controlling either the pod or flag on the pad)

[razark]

Put a probe out in a high orbit. I've got one at 83 million or so, right at the edge of Kerbin's SOI. Put yourself in whatever parking orbit you want, switch to the probe, and timewarp all you want.

[The Ideal Gas Lawyer]

That's quite the large parking orbit. I don't know how much the Oberth effect would change, but I do know that if you set your orbit at about 200km and then set a pod on the pad (Or have a flag placed at the pad) you can switch to map view and warp all you want (While controlling either the pod or flag on the pad)

Put a probe out in a high orbit. I've got one at 83 million or so, right at the edge of Kerbin's SOI. Put yourself in whatever parking orbit you want, switch to the probe, and timewarp all you want.

That's usually what I end up doing. Its just easy to overshoot. I often use MJ to plan maneuvers even if I don't use it to perform them and when I switch to another entity I can't see the "time to node". Also it deletes the node.

[razark]

Kerbal Alarm Clock:

http://forum.kerbalspaceprogram.com/threads/24786

[The Ideal Gas Lawyer]

Probably what I'll do. Still wondering about how much fuel I'm wasting, though

[SnappingTurtle]

http://en.wikipedia.org/wiki/Oberth_effect

According to the equation on this page the rate of change of kinetic energy is proportional to velocity. So if you're in a lower orbit that has you moving twice as fast as a higher orbit you'll have to burn your engines for half the time to achieve the same change in kinetic energy.

Disclaimer: I am not a mathematician or an astrophysicist.

[Deathsoul097]

You'll be wasting a few hundred ms-1, but if you lower your periapsis closer to the planet (To about 75km/80Km) from your parking orbit, you will end up expending less than a direct transfer from that parking orbit. It requires a bit more planning than normal, but it should be easy to do.

[numerobis]

Let's say you need to leave the Kerbin SOI at 1km/s (we're going to Eve today). Let's compare leaving from a 70km orbit, versus leaving from a 650km orbit. But remember you had to pay to get to 650 km, so let's add the cost of the transfer from 70 to 650.As follows:

>>> import planet
>>> planet.kerbin.soiBurn(70000, 1000)
1089.1152384926713
>>> planet.kerbin.soiBurn(650000, 1000)
881.6874035931148
>>> planet.kerbin.soiBurn(650000, 1000) + sum(planet.kerbin.hohmann(70000, 650000))
1482.2548208458336

So the first option is about 400 m/s cheaper in the end. Leaving straight from 650 km is cheaper, but you had to pay to get there.

If you bump up the speed to 2000 m/s (going to Jool), you get a similar answer: from 70km is 500 m/s cheaper overall.

[Kasuha]

You're not wasting anything. In fact, you're saving. Getting there from higher orbit costs less dv than getting to the same place from lower orbit.

You can try it yourself. The default transfer in the launch window planner costs 1696 m/s, if you change the starting orbit to 605 km, the result is 1507 m/s.

The only place where you are technically wasting fuel is when you are lifting things there from Kerbin surface. Because it costs you more fuel when getting there.

[Rhomphaia]

You're not wasting anything. In fact, you're saving. Getting there from higher orbit costs less dv than getting to the same place from lower orbit.

You can try it yourself. The default transfer in the launch window planner costs 1696 m/s, if you change the starting orbit to 605 km, the result is 1507 m/s.

The only place where you are technically wasting fuel is when you are lifting things there from Kerbin surface. Because it costs you more fuel when getting there.

And it takes a fair bit more than 189 delta v to get to that 605km orbit rather than the 100km.

Also try it for Eeloo. cost 11m/s more to go from 605km than it does to go from 100km.

[Kasuha]

And it takes a fair bit more than 189 delta v to get to that 605km orbit rather than the 100km.

You're right. I assumed you can refill fuel on that waiting orbit. Because refuelling stations are relatively cheap while refuelling on mission takes another mission.

Also try it for Eeloo. cost 11m/s more to go from 605km than it does to go from 100km.

Jool also comes out cheaper by 3 m/s I think.

[capi3101]

Short answer - the Oberth effect off of Kerbin isn't really that significant to begin with. I did the calculations on another thread not too long ago. IIRC, even at 70k (as low as you can go without hitting Kerbin's atmosphere), the Oberth effect only saves you about 300 m/s of delta-V.

I'll have to find that thread again and see what you get at 605k.

[The Ideal Gas Lawyer]

You're not wasting anything. In fact, you're saving. Getting there from higher orbit costs less dv than getting to the same place from lower orbit.

You can try it yourself. The default transfer in the launch window planner costs 1696 m/s, if you change the starting orbit to 605 km, the result is 1507 m/s.

The only place where you are technically wasting fuel is when you are lifting things there from Kerbin surface. Because it costs you more fuel when getting there.

That assumes I’m refueling at that orbit which is not the case. I suppose I could but if I can launch straight to my target (ie if I don’t *need* to gas up) I tend to do that.

[el_coyoto]

I'd say it depends on where you wish to go after you parked you craft...

If you don't refuel at a station before burning, the lower, the better because of Oberth, although I don't really answer your question of "how much better?".

If you plan on refueling your ship at a station before going places, the following post contains the ideal hohman transfer altitudes at which to put the refueling station : Efficient Hohmann Transfer Altitudes.

What the above means is that there exists an optimal altitude for most interplanetary transfer burns. This in only useful if you have put a refueling station at that altitude : you will gain nothing (even loose) if your craft has to cut through the climbing with its own fuel because of the lower Oberth effect. It also means that you will lose a bit fuel from your station+ship setup (less Oberth effect) but your ship will arrive at its target with slightly more fuel.

[blizzy78]

It's worth noting that you don't need to get into a high parking orbit so that you can use max timewarp. You can always max timewarp with any landed vessel, including one still being on the pad.

[Kasuha]

It's worth noting that you don't need to get into a high parking orbit so that you can use max timewarp. You can always max timewarp with any landed vessel, including one still being on the pad.

Or using any of your flags, if you have any.

[CaptainArchmage]

It requires more delta-V to get into a 605km parking orbit, so I think you can reduce your parking orbit to somewhere between 250km and 400km. This makes docking easier and it often provides better FPS than using an orbit of 80-100km, which is really more important than reducing delta-V in KSP at least at the moment.

[The Ideal Gas Lawyer]

It's worth noting that you don't need to get into a high parking orbit so that you can use max timewarp. You can always max timewarp with any landed vessel, including one still being on the pad.

Or using any of your flags, if you have any.

I addressed this earlier in the thread

I'd say it depends on where you wish to go after you parked you craft...

If you don't refuel at a station before burning, the lower, the better because of Oberth, although I don't really answer your question of "how much better?"

Exactly. It might help if I make the question more specific. If I am going to Jool, What is the difference in dV between these two flight plans:

1) Launch from the surface of Kerbin into a 100km parking orbit, wait for the TJI window and burn until I get an intercept

2) Launch from the surface of Kerbin into a 605km parking orbit, wait for the TJI window and burn until I get an intercept

[Rhomphaia]

[quote name=The Ideal Gas Lawyer;829203

1) Launch from the surface of Kerbin into a 100km parking orbit' date=' wait for the TJI window and burn until I get an intercept

2) Launch from the surface of Kerbin into a 605km parking orbit, wait for the TJI window and burn until I get an intercept

[Tarrow]

Exactly. It might help if I make the question more specific. If I am going to Jool, What is the difference in dV between these two flight plans:

1) Launch from the surface of Kerbin into a 100km parking orbit, wait for the TJI window and burn until I get an intercept

2) Launch from the surface of Kerbin into a 605km parking orbit, wait for the TJI window and burn until I get an intercept

Just done a couple of back to back checks using one of my probe setups.

Ground to 100km orbit required 4604m/s delta, transfer to jool requires 1926m/s

Ground to 605km orbit required 5137m/s delta, transfer to jool requires 1918m/s

It's more efficient from the lower orbit (by 525m/s). Orbital altitude makes practically no difference to the required delta for the interplanetary burn. However the 101 so days until my transfer window would be best spent at the highest possible time acceleration. I tend to park my interplanetary stuff at 1250km to cut down on actual waiting time, I'm too disorganised to do the time accel whilst landed.

[Xephirious]

Another thing to consider is when using LV-N nuclear and Ion engines burn times can be quite long. If you start a burn 10 mins before AP in LKO you will end up deorbiting the craft if you are using maneuvre nodes. A higher parking orbit reduces this effect.

I know it's possible to leave LKO to Jool using LV-N but you'd better not aim at the blue nav ball target because you're gonna de orbit your space train.

I personally use a 605km construction orbit for all interplanetary craft for the same reason as OP. Just keep a refueler rig in orbit and time launches to intercept at 600km.

Go straight to 600km then circularize, don't circularize at 100k then raise your AP as it costs more fuel.

[The Ideal Gas Lawyer]

Just done a couple of back to back checks using one of my probe setups.

Ground to 100km orbit required 4604m/s delta, transfer to jool requires 1926m/s

Ground to 605km orbit required 5137m/s delta, transfer to jool requires 1918m/s

Well the difference in the launch between a 75km orbit and a 605km orbit is around 310

the first window to Jool the difference in ejection burn is 3 in favour of the higher orbit

So you lose 307 deltaV with the second option.

[Anton P. Nym]

Another thing to consider is when using LV-N nuclear and Ion engines burn times can be quite long. If you start a burn 10 mins before AP in LKO you will end up deorbiting the craft if you are using maneuvre nodes. A higher parking orbit reduces this effect.

I use an eccentric parking orbit for this reason; periapsis at around the 120km mark, apoapsis out scraping my geosynchronous satellites. It makes for a roughly 3 hour orbit, so not too long to hit a window, and gets about 3500m/s down at the bottom to take advantage of Herr Oberth's math.

-- Steve

[The Ideal Gas Lawyer]

I use an eccentric parking orbit for this reason; periapsis at around the 120km mark, apoapsis out scraping my geosynchronous satellites. It makes for a roughly 3 hour orbit, so not too long to hit a window, and gets about 3500m/s down at the bottom to take advantage of Herr Oberth's math.

-- Steve

That's got to put a real cramp in your timewarp though. Also aligning your major axis such that its points prograde At the time you wish to escape might be kind of difficult